Because of their excellent thermal stability, low temperature flexibility and high dielectric strength, silicones have been used for coating, potting or encapsulating electrical devices such as integrated circuits. Potting or encapsulating materials typically used are elastomeric or gel-like materials providing shock, vibrational and thermal stress protection on fragile electronic parts. References relating generally to silicone gel encapsulants include U.S. Pat. Nos. 3,933,712; 4,072,635; 4,087,585; 4,271,425; 4,374,967; and D. Dickson, Jr., proceedings Electric/Electronic Intel. Conf. 12, 92 (1975). See also prior art description in U.S. Pat. No. 4,374,967.
The present commercially available potting silicones include one component compositions such as U.S. Pat. No. 4,271,425 which are cured by moisture, requiring several hours to days to complete the cure. Faster cures may be obtained from two component systems such as those in U.S. Pat. No. 4,087,585 if elevated temperatures are provided. The two component systems which require a platinum catalyst, however, are inhibited by organotin compounds, sulfur, amines, urethanes and unsaturated hydrocarbon plasticizers on the substrate surface. Curable silicone formulations have also been used for providing crosslinked environmentally resistant coatings on articles such as electronic circuit boards and the like which include raised parts such as resistors, transistors, micro chips, etc.
It is known that certain silicones containing methacrylate or acrylate functional groups may be cured by ultraviolet light. U.S. Pat. Nos. 4,201,808 and 4,348,454 and U.K. Patent application No. GB2039287A all describe U.V. curing compositions of silicones containing methacrylates or acrylates for paper release coatings. Such materials, however, have not been used on substrates, such as electronic circuit boards, which include light inaccessible or "shadow" areas because the coating compositions cannot be cured in these areas by ultraviolet light.